Machine tools

Machine tools

1. 13.305 MACHINE TOOLS

2. FUNDAMENTALS OF METAL CUTTING
Overview of Machining Technology
Theory of Chip Formation in Metal Machining
Tool geometry
Force Relationships and the Merchant Equation
Factors affecting tool life
Power and Energy Relationships in Machining
Cutting Temperature
Tool inserts Specifications
Cutting fluid


2

3. MACHINING PROCESSES AND MACHINE TOOLS
Parts can be manufactured by casting,
forming and shaping processes


Manufacturing process can be classified
as

Additive Manufacturing Process
Subtractive Manufacturing Process

They often require further operations
before the product is ready for use
Copyright © 2010 Pearson Education South Asia Pte Ltd

4. MANUFACTURING PROCESS
Additive
Manufacturing (AM)
is an appropriate
name to describe the
technologies that
build 3D objects by
adding layer-upon-
layer of material

Subtractive
manufacturing is a
process by which 3D
objects are
constructed by
successively cutting
material away from a
solid block of material.
4

5. MACHINING PROCESSES
5

The term Machining means, the removal of excess material and
modification of the surfaces of a blank.

Machining is the process by which
a work piece is given a desired shape, size and surface finish
by removing the excess material from the work piece
with the help of a properly shaped cutting tool.


Copyright © 2010 Pearson Education South Asia Pte Ltd

6. A machine tool is a power driven cutting machine which is used for
shaping, sizing or processing a work piece to a product of desired
accuracy by removing excess material from the surface in the form
of metal chips.
e.g. lathe, Shaping machine, planning machine, slotting machine,
milling machine, drilling machine, grinding machine etc.
Machine Tools
6

7. FUNCTIONS OF MACHINE TOOL

Functions of a machine tool are

Holding, supporting and guiding the work piece to be machined.

Holding, supporting and guiding the cutting tool.

Regulating the cutting speed and movement between work piece and
cutting tool.

Providing the required motion to the work piece and cutting tool.

Performing various operations required.

7

8. During machining the cutting tool exerts a compressive
force on the workpiece and when this force exceeds the
yield point, the material starts to deform plastically.

Figure 21.2 - (a) A cross-sectional view of the machining process, (b)
tool with negative rake angle; compare with positive rake angle in (a)

9. ORTHOGONAL & OBLIQUE CUTTING
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10. ORTHOGONAL CUTTING
2-D cutting process.
Only two components of
cutting force are there
Cutting edge is perpendicular
to the direction of cutting
speed
Chip flows over the face of
tool.(normal to cutting edge)
Max chip thickness occurs at
the middle
Tool is perfectly sharp and
contacts the chip on rake face
only
Only one cutting edge is in
action
Tool life is less

11. OBLIQUE CUTTING
3-D cutting process.
Three components of
cutting forces are there
Cutting edge is at an angle
to the direction of cutting
speed
Chip flows over the face of
tool.(at an angle to cutting
the edge)
Max chip thickness may or
may occurs at the middle
More than one cutting edge
is in action,
Tool life is more

12. CHIP FORMATION
oUnwanted material removed from the workpiece is known
as Chip.
oThe basic requirement for the chip formation are
oThe tool must be harder than workpiece material
oThere must be a relative motion between tool and workpiece
Types of chips
1.Discontinuous chip
2.Continuous chip
3.Continuous chip with Built-up Edge (BUE)

12

13. Ductile work materials (e.g., low
carbon steel)
Small depth of cut, large rake
angle , presents of cutting fluid,
High cutting speed, small feed rate
Chips are in the form of long coil .
Uniform thickness
Sharp cutting edge on the tool
Low tool-chip friction.
Good surface finish.
High tool life.
Less power consumption
Chip disposal is a problem




CONTINUOUS CHIP
13

14. Continuous with BUE

Ductile materials
Small rake angle , low to
medium cutting speed& absence
or insufficient cutting fluid
Chip get welded on to tool /work
surface
High adhesion causes portions of
chip to adhere to rake face
BUE formation is cyclical; it
forms, then breaks off
(c) continuous with built-up
edge
14

15. Discontinuous chip

Brittle work materials (e.g., cast
iron,bronze,)

Low cutting speed, and no
coolant.

Small rake angles

Large feed and depth of cut

High tool-chip friction

This chip can be easily handled






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16. 16

17. VARIABLES INFLUENCING CHIP FORMATION
 Mechanical properties of material to be cut
 Depth of cut
 Cutting speed
 Feed rate
 Type of cutting fluid
 Machining temperature at cutting region
Coefficient of friction between chip and tool
17

18. CUTTING TOOL CLASSIFICATION
1.Single-Point Tools
One cutting edge
Turning uses single point tools
Point is usually rounded to form a nose radius

2.Multiple Cutting Edge Tools
More than one cutting edge
Motion relative to work usually achieved by
rotating
Drilling and milling use rotating multiple cutting
edge tools.
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19. Figure 21.4 - (a) A single-point tool showing rake face, flank, and tool
point; and (b) a helical milling cutter, representative of tools with
multiple cutting edges
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20. PARTS OF SINGLE POINT CUTTING TOOL
Shank
it is the body of the tool, on one
end of which cutting point is
formed
Face
The surface over which the
chip impinges as it is removed
from workpiece
Flank
The surface of the tool which is
facing the work piece

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21. Base
Bottom surface of shank.
Nose
It is the curve formed by
joining the side cutting
edge and end cutting edge.
Cutting edge
The portion of the face edge
along which the chip is
separated from the work



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22. TOOL GEOMETRY
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23. TOOL GEOMETRY
Back rake angle:
It is the angle b/w the face of
the tool and the line parallel to
the base of the tool
Measured in a plane parallel
to the centre line of point
and right angles to the base.
+ve = face slopes downward
-ve = face slopes upward.
It controls the formation of
chip and guides its direction
of flow


23

24. Side rake angle:
 It is the angle b/w the tool face and line parallel to
the base of the tool
 Measured in a plane perpendicular both the centre
line of point and the base.
6-15⁰
It also controls the direction of chip flow




24

25. Relief Angle:
 It is the angle b/w a plane perpendicular to the base
of a tool and the flank immediately adjacent to cutting
edge.
It controls the rubbing at tool-workpiece interface.
Higher the relief angle ,the tool may be chip-off
Smaller the relief angle, greater will be the flank wear.
(a) side relief angle
(b) end relief angle
Side relief angle
Angle b/w the portion of the flank immediately below
the cutting edge and a line drawn through the cutting
edge/point .



25

26. End relief angle
Angle b/w the portion of the end flank immediately
below the cutting edge and a line drawn through the
that cutting edge perpendicular to base.
8-15⁰





26

27. Clearance (cutting)angle
oAngle between a plane perpendicular to the base of a
tool and that of flank immediately adjacent to the
base.






End clearance angle
Angle between a plane perpendicular to the base of a
tool and that of end flank(surface below the end
cutting edge) immediately adjacent to the base
Side clearance angle
Angle between a plane perpendicular to the base of a
tool and that of side flank(surface below the cutting
edge) immediately adjacent to the base

27

28. Nose radius
It is the curve formed by joining side cutting edge and
the end cutting edge.








Larger the nose radius, greater will be the surface
finish.
28

29. TOOL SIGNATURE
Tool angles have been standardized by the American
Standard Association (ASA)
Tool angles given in a definite pattern
Back rake angle-
Side rake angle-
End relief angle
Side relief angle-
End cutting edge angle 8
Side cutting edge angle
Nose radius – 0.8 mm
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30. PROBLEM
A tool having 8, 8, 5, 5, 6, 6, and 1 as signature
in ASA system will have the following angles.
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31. MACHINABILITY
MT
Machinability is the easiness with which a material can
be machined satisfactorily.

Good machinability refers to removal of material with
moderate cutting forces.

Variables affecting machinability are
Work variable
Tool variable
Machine variable
Cutting conditions

32. 32
Work variables
Chemical composition
Micro structure
Mechanical Properties
Physical properties
Method of manufacturing
32
Tool variables
Tool geometry
Nature of cutting
Rigidity of tool

33. 33
Machine variables
Rigidity of machine
Power and accuracy of M T
oCutting conditions
oCutting speed has greatest influence on tool life and
machinability.

ADVANTAGES
•Good surface finish
•High cutting speed
•Less power consumption
•High MRR
•Less tool wear

34. TOOL LIFE: WEAR AND FAILURE
34
Tool wear is gradual process; happening due to:
1.High localized stresses at the tip of the tool
2.High temperatures (especially along rake face)
3.Sliding of the chip along the rake face
4.Sliding of the tool along the newly cut work piece
surface
The rate of tool wear depends on
Tool and workpiece materials
Tool geometry
Process parameters
Cutting fluids
Characteristics of the machine tool
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35. 35
Tool wear and the changes in tool geometry are classified
as:
a)Flank wear
b)Crater wear
c)Nose wear
d)Thermal cracking
e)Flank wear with BUE


Copyright © 2010 Pearson Education South Asia Pte Ltd

36. FLANK WEAR
This is also called edge wear.
Friction/abrasion/adhesion, b/w tool & workpiece are
the main causes of flank wear.
It is the flat worn out portion on flank, and called
wear land.
Flank wear takes place when machining brittle
materials like cast iron.
It also occurs when the feed is less than
0.15mm/revolution.
As the flank wear increases, the temperature at
tool/workpiece also increases.
Due to this the hardness of the tool decreases and it
fails.
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37. 37

38. CREATER WEAR
It occurs at face of the tool.
As chip slides over the face of the tool, it worn out
gradually.
The cavity formed on the face of the tool is known as
creater wear.
As the creater become large, the cutting edge may
break from tool.
It occurs when machining ductile material.


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39. 39
a) Features of tool wear in a turning operation. VB: indicates average flank
wear
b) – e)
Examples of
wear in
cutting tools
b)
Flank
wear
c)
Crater
wear
d)
Thermal
cracking
e) Flank
wear and
built-up
edge (BUE)

40. FACTORS INFLUENCING CRATER WEAR
40

1.Temperature at the tool–chip interface
2.Chemical affinity between tool and workpiece
materials
Crater wear occurs due to “diffusion mechanism”
This is the movement of atoms across tool-chip interface
Since diffusion rate increases with increasing
temperature, ⇒ crater wear increases as temperature
increases
Note how quickly crater wear-rate
increases in a small temperature
range
Coatings to tools is an effective
way to slow down diffusion process
(e.g. titanium nitride, alum. oxide)

41. OTHER TYPES OF WEAR
41
Nose wear is the rounding of a sharp tool due to
mechanical and thermal effects

It dulls the tool, affects chip formation, and causes rubbing of
the tool over the work piece
This raises tool temperature, which causes residual stresses
on machined surface

Tools also may undergo plastic deformation because
of temperature rises in the cutting zone

Thermal cracking happens due to elevated temperature
at tool work piece interface.

42. CHIPPING, AND FRACTURE
42
Tools may undergo chipping, where small fragment
from the cutting edge of the tool breaks away
Mostly occurs with brittle tool materials (e.g. ceramics)
Small fragments: “microchipping” or “macrochipping”
Large fragments: “gross fracture” or “catastrophic failure”
Chipping may occur in a region of the tool where a
small crack already exists
This causes sudden loss of tool material, change in tool shape
⇒ drastic effects on surface finish, dimensional accuracy
Two main causes of chipping
Mechanical shock (impact due to interrupted cutting)
Thermal fatigue (variations in temp. due to interrupted
cutting)

43. 43
It is very important to continuously monitor the
condition of the cutting tool to observe wear, chipping,
gross failure
Classified into 2 categories:
1.Direct method
2.Indirect methods


WEAR MEASUREMENT

44. 44
1.Direct method for observing the condition of a cutting tool
involves optical measurements of wear
e.g. periodic observation of changes in tool using tool maker’s
microscope
e.g. programming tool to touch a sensor after every machining cycle
(to detect broken tools)

45. 45
2.Indirect methods of observing tool conditions involve the
correlation of the tool condition with certain parameters
Parameters include forces, power, temp. rise, surface finish,
vibration, chatter
e.g. transducers which correlate acoustic emissions (from stress
waves in cutting) to tool wear and chipping
e.g. transducers which continually monitor torque and forces
during cutting, plus measure and compensate for tool wear
e.g. sensors which measure temperature during machining

46. FACTORS AFFECTING TOOL LIFE
The life of the cutting tool is affected by

Cutting speed
Feed & depth of cut
Tool geometry
Tool material
Cutting fluid
Work material
Rigidity of work, tool, and machine

46

47. CUTTING SPEED
When cutting speed increases, the tool life
decreases because cutting speed has greater
influence on the MRR and thereby tool life.

 when cutting speed increases the tool/workpiece
interface temperature will also increase and as a
result hardness of tool decreases.

 The relation b/w cutting speed and tool life in
terms of Taylor’s formula

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48. 48
Taylor tool life equation :





Copyright © 2010 Pearson Education South Asia Pte Ltd CVT
n

V = cutting speed [m/minute]
T = tool life in minutes
n = an exponent or index which depends on tool and work
C = constant. It is numerically equal to cutting speed that gives a tool life of one
minute.

49. FEED AND DEPTH OF CUT
The life of the cutting tool is influenced by the
amount of metal removed by the tool per minute
 The effect of feed and depth of cut on tool life is
given by
V= 257 / (T^0.19*f^0.36*t^0.8) m/min

V= cutting speed
T= tool life
f = feed in mm/min
t = depth of cut in mm

49

50. TOOL MATERIAL
An ideal tool is one which remove max volume of
material at all cutting speed

Mechanical , Physical and chemical properties of tool
material will influence tool life

Carbide tools have more life than high speed steel.

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51. CUTTING FLUID
Heat produced during metal cutting is carried away
from the tool and work by means of cutting fluid

Cutting fluid reduces friction at chip tool interface and
increases tool life.

Cutting tool which directly control the amount of heat
at the chip tool interface is given by the formula
TӨ^n= C
T= tool life
Ө = temp @chip tool interface
n= index which depends upon the shape and material
of the cutting tool.

51

52. WORK PIECE MATERIAL
Tool life also depends on the micro structure of the
work piece material.

 Tool life will be more when machining soft metals
than hard metals like cast iron and alloy steel.

52

53. RIGIDITY OF WORK , TOOL , AND MACHINE
A strongly supported tool on a rigid machine will
have more life than tool machining under
vibrating machine.

 loose work piece will decrease the tool life.
53

54. FORCES ON A SINGLE POINT CUTTING TOOL
Work material offers resistance during machining.

 This resistance is overcome by cutting force applied to tool
face/ point.

Work done by this force causes the deformation of the
metal in the form of chip.

Magnitude of this cutting force depends upon
Material being machined
Feed rate
Depth of cut
Tool geometry
Cutting speed
Coolant used

54

55. MEASUREMENT OF CUTTING FORCES

DIRECT METHOD
Mechanical dial gauges
Strain gauge dynamometer
Pneumatic and Hydraulic dynamometers
Electrical Dynamometers
Piezo-electrical dynamometers

INDIRECT METHODS
With the aid of watt meter
By measuring variation of voltage & current consumption during
machining.
55

56. NEED FOR CUTTING FORCE DETERMINATION
Determination of power consumption.

Selection of motor.

Structural design of Machine.

Maximize productivity
56

57. 57

58. 58

59. 59

60. MERCHANT’S CIRCLE
Introduced by Earnst Merchant.

This theory assumes that the cutting is orthogonal and
shear angle is located where the energy required for the
deformation is minimum, also the workdone is
minimum.

This is known as the minimum energy theory

60

61. MERCHANT’S CIRCLE DIAGRAM
The following is a circle diagram Known as Merchant’s
circle diagram, which is convenient to determine the
relation between the various forces and angles.
61

62. In the diagram two force triangles have been combined
and R and R’ together have been replaced by R. the force
R can be resolved into two components Fc and Ft.

Fc and Ft can be determined by force dynamometers

The rake angle (α) can be measured from the tool, and
forces F and N can then be determined. The shear angle
(φ) can be obtained from it’s relation with chip reduction
coefficient.

Now Fs & Fn can also be determined
62

63. ASSUMPTIONS
Work moves with uniform velocity.
The shear is occurring in a plane.
The tool is perfectly sharp and no contact along clearance
face.
The cutting edge is a straight line.
Width of tool is greater than width of work piece
Stress on the shear plane is uniformly distributed.
Uncut chip thickness is constant.
Continuous chip is produced with no built up edge.
The chip does not flow to either side.
63

64. THE PROCEDURE TO CONSTRUCT A MERCHANT’S
CIRCLE DIAGRAM
Set up x-y axis labeled with forces, and the origin in
the centre of the page. The cutting force (Fc) is drawn
horizontally, and the tangential force (Ft) is drawn
vertically. (Draw in the resultant (R) of Fc and Ft
64
Ft
U
M
Fc
L

65. 65
R
Locate the centre of R, and draw a circle that encloses
vector r. if done correctly, the heads and tails of all 3
vectors will lie on this circle
R

66. Draw in the cutting tool in the upper right hand
quadrant, taking care to draw the correct rake angle
(α) from the vertical axis.
66
α

67. Draw the cutting tool in the upper right hand quadrant,
taking care to draw the correct rake angle (α) from the
vertical axis
67
F
P

68. A line can now be drawn from the head of the friction vector,
to the head of the resultant vector (R). This gives the normal
vector (N). Also add a friction angle (β) between vectors R and
N. Therefore, mathematically, R = Fc +Ft= F +N.
68
N
β

69. Draw a feed thickness line parallel to the horizontal axis.
Next draw a chip thickness line parallel to the tool
cutting face
69
Feed thickness line
Chip thickness line
α
φ
S
α
Fs

70. Draw a vector from the origin (tool point) towards the
intersection of the two chip lines, stopping at the circle.
The result will be a shear force vector (Fs). Also measure
the shear force angle between Fs and Fc
70
Fn

71. Finally add the shear force normal (Fn) from the head
of Fs to the head of R.
Use a scale and protractor to measure off all distances
(forces) and angles.
71
U
α
L
W
Β-α
φ
90- α
90- β
z

72. 72
Q
M
U
α
φ
Z

73. 73
90-φ
φ
U L
Q
Z

74. 74

75. 75

76. 76

77. 77

78. 78

79. 79

80. 80

81. 81

82. 82

83. 83

84. ADVANTAGES OF MERCHANT’S CIRCLE
DIAGRAM
Easy , quick and reasonably accurate
determination of several other forces from a few
known forces involved in machining.
Friction at chip tool interface and dynamic yield
shear strength can be easily determined.
Equation relating the different forces can be
easily developed
84

85. LIMITATIONS OF MCD
MCD is valid only for orthogonal cutting

By the ratio F/N, the MCD gives apparent (not
real)coefficient of friction.

It is based on single shear plane theory.
85

86. ECONOMIC OF MACHINING
It is used to obtain lowest possible unit cost and highest
possible production rate for any given operation.

At highest cutting speed , the tool cost may increase owing
to shorter tool life , and the tool cost per unit piece decrease.

COST PER PIECE = IDLE COST PER PIECE+TOOL
CHANGING COST PER PIECE+ + CUTTING COST PER
PIECE


86

87. CUTTING COST PER
PIECE.

Cutting cost per
piece depends on the
time , the tool
actually cut the
workpiece
It can be reduced by
increasing MRR
87

88. IDLE COST PER PIECE
This includes the
time spent in loading
and unloading the
piece and the tool
approach time.
It can be reduced by
using jigs & fixtures,
centralized machining
concept etc.

88

89. TOOL CHANGING COST PER PIECE





This includes operators
time to change the tool
and to grind it.

89

90. This includes
depreciation of tool and
the cost of grinding.
TOTAL COST CURVE IS
THE SUM OF ALL
INDIVIDAUL CURVES.
90
TOOL REGRINDING COST PER PIECE

91. TOOL INSERT SPECIFICATION
Tool insert
Uncoated tungsten carbide tool-CNMG 12 04 08 H13 A
Make: Sandvik Coromant.


91
C Insert shape (C=80˚)
N Insert clearance angle (N=0˚)
M Tolerance ± on thickness (s)
G Insert type
12 Insert size (cutting edge l2 mm)
04 Insert thickness, s (04mm)
08 Insert radius, r
ɛ mm

92. 92

93. 93

94. CUTTING FLUIDS
To improve machinability , any substance applied to
the cutting zone during machining is called cutting
fluids.
Cutting fluids can act as coolant and as lubricant

A cutting fluid used to cool the tool and work piece is
called coolant
Water based coolants.

A cutting fluid used for the purpose of diminishing
friction between contacting surface in the cutting
zone is called lubricants.
Oil based fluids


94

95. FUNCTIONS OF CUTTING FLUIDS
1.To carry away the heat generated at work-tool
interface.
Tool hardness maintained
Less tool wear
Longer tool life
2.To reduce the friction at work tool interface.
Less power consumption.
Less heat generation

3.To flush away the chip from the tool.
4.To protect the finished surface from corrosion.
5.To break up the chip into small pieces.
6.To prevent formation of built-up edge.
7.To improve surface finish.








95

96. PROPERTIES OF CUTTING FLUIDS
It should posses good lubricating properties to minimize
friction at tool/work piece interface.
It should posses high heat absorption capacity.
It should not produce any skin irritation to the operator.
They should not emit obnoxious odours and vapours, harmful
to operator.
It should have less viscosity
It should be transparent.
It should be easily available at low price.
It should be chemically stable.
It should have high flash point.




96

97. TYPES OF CUTTING FLUIDS
A) Soluble oils (emulsions)
Water based cutting fluids
Mineral oil is dispersed in the form of fine droplets.
Oil & Water mixed in different proportions to get desired
properties.
Ratio varies from 1 : 5 to 1 : 50
Suitable for light cutting operations.

97

98. B) Straight oils
It is a mineral oil with suitable viscosity.
It has improved lubricating properties over soluble oils.
It maintains the lubricating film at low pressure.

98

99. C) Chemical additives
Additives such as sulphur and chlorine are used to
increase both the cooling & lubricating properties of oil.
It can maintain the lubrication film at extreme pressure.
Prevents the formation of built-up edge.
Suitable for machining low carbon steel.
D) Chemical compounds
Rust inhibitors such as sodium nitrate is mixed with high
percentage of water to obtain chemical compounds.
It prevents rust formation on machined surface.
Suitable for grinding operation

99

100. Solid Lubricants
Stick waxes, bar soaps and graphite powder are sometimes
used as solid lubricants.
100

101. SELECTION OF CUTTING FLUIDS
 Cutting speed
 Feed rate
Depth of cut
Tool and workpiece material
Viscosity of cutting fluid
 Tool life to be expected
 Economical aspects
 Life of cutting fluids

101

102. CHIP BREAKERS
Long continuous chip are undesirable.

Chip breaker is a piece of metal clamped to the rake
surface of the tool which bends the chip and breaks it.

Chips can also be broken by changing the tool geometry,
thereby controlling the chip flow

Types
Step type
Groove type
Clamp type
102

103. Step type
A step is formed on the tool face behind the cutting edge.
Groove type
A groove is provided on the face behind the cutting edge.
Clamp type
A thin piece of material (chip breaker) is clamped or screwed
on the face of the tool.

103

104. CUTTING TOOL MATERIALS
Hot hardness
The material should remain harder than the work material at
elevated operating temperatures.
Wear resistance
The material must withstand excessive wear even though the
relative hardness of the tool-work material changes.
Toughness
The material must have sufficient strength to withstand shocks and
vibrations and to prevent breakage.
Cost and easiness in fabrication
The cost and easiness of fabrication should have within reasonable
limits.

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105. Different types of cutting tool materials ;

Carbon steel
High speed steel
18-4-1 HSS
Molybdenum HSS
Cobalt HSS
Cemented carbides
Ceramics
Diamonds

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106. Carbon steel

Plain carbon steel containing
Carbon - 0.8 – 1.3 %
Silicon 0.1 – 0.4 %
Manganese – 0.1 – 0.4 %
Suitable for low cutting speeds and cutting temperature
less than 200⁰c.
At heat treated & tempered condition this steel have
sufficient hardness, strength and toughness.
Heat treatment is done to provide keen cutting edge
This material is cheap , easy to forge and simple to
harden.
Suitable for
Taps & dies
Reamers
Hacksaw blades
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107. HIGH SPEED STEEL
These tools can cut the material efficiently at high
speed.(2 to 3 times if carbon steel)
It has superior hot hardness and high wear resistance.
It maintains its hardness up to 900⁰C.
The various alloying elements added to improve its hot
hardness and wear resistance are
Tungsten
Chromium
Vanadium
Cobalt
molybdenum
Types of HSS
18-4-1 HSS
Molybdenum HSS
Cobalt HSS
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108. 18-4-1 HSS
Tungsten - 18%
Chromium – 4 %
Vanadium – 1 %
Carbon – 0.75 %
Most commonly used tool steel.
Molybdenum HSS
Tungsten – 5 %
Chromium – 4 %
Vanadium – 2 %
Molybdenum – 6 %
It has high toughness and cutting ability.
Cobalt HSS
Tungsten – 20 %
Chromium – 4 %
Vanadium – 2 %
Cobalt – 15 % - (to increase hot hardness)
It is used for heavy duty and rough cutting tools in planers and milling
machines.
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109. CEMENTED CARBIDES
Cemented carbides are made by mixing tungsten powder
and carbon at high temperature (1500⁰C) in the ratio 94 :
6 by weight.
Then it is combined with cobalt ,compacted and sintered
in a furnace about 1400 ⁰ C
It can be operated at higher cutting speed.


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110. CERAMICS
Aluminium oxide and boron nitride powders are
mixed together and sintered at1700⁰C to form the
ingredient of ceramic tools.

It has high hardness and compressive strength.

It is made as tips and brazed/clamped on to the metal
shank for cutting


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111. DIAMOND
Hardest cutting tool material.
It can be run at a speed 50 times greater than HSS
It can be made artificially by sintering at very high
pressure and temperature.
It has low coefficient of friction.
High compressive strength and wear resistance.
Low coefficient of thermal expansion.
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112. 112

113. SELECTION OF DIFFERENT WORK
PIECE MATERIALS
Hardness

Abrasive qualities

Toughness

Tendency to weld

Inherent hard spot and surface inclusion.
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